Nuclear magnetic resonance imaging has been used in the medical art for some time as a clinical tool for diagnosis of the diseases and maladies that affect the human body. In many applications, NMR imaging is considered by those skilled in the art as the method of choice over other types of imaging methods such as "X-Ray" imaging or acoustical imaging. NMR imaging provides a non-invasive and efficient method of imaging certain areas of a patient's body in certain applications.
The NMR technique is well known. The sample to be imaged is subjected to an external magnetic field, B.sub.0. Since the human body is primarily composed of water, the protons in the water molecules align themselves with the external magnetic field. The protons have magnetic dipole moments and possess spin quantum numbers with multiplicity of 1/2. An alternating magnetic field is applied to the system with frequency w.sub.0 which is close to the Larmor frequency of the protons. Due to the spin quantum numbers associated with the protons, they will resonate at the frequency of the alternating field. Resonance occurs since the alternating magnetic field tends to force the protons to oscillate around the direction of the field lines of the external magnetic field, B.sub.0, in an effort to realign with the;external magnetic field. During oscillation the protons emit radio frequency (rf) signals which are termed "NMR signals".
The NMR phenomenon can be viewed as a splitting of nuclear Zeeman sublevels due to the multiplicity of allowed spin quantum numbers among the protons. Strong oscillation occurs at the resonant frequency, w.sub.0, which can be detected if a suitable receiving mechanism is placed on or near the sample which is under examination. Generally, a coil is used to receive the signals from the sample as it undergoes oscillation. There has been great interest in the NMR art to design improved coils for high resolution imaging of all of the desired areas of the human body. An important motivating factor in NMR coil design has also been to ensure ease of use coupled with a high degree of patient comfort.
In the past, coils have been designed with a high degree of stiffness. Typically, the NMR coil is viewed as an antenna system with a plurality of coil elements comprising the system. Such an arrangement is embodied for example in U.S. Pat. No. 4,620,155 to Edelstein. The coil arrangement in Edelstein is of necessity configured as a rigid body. Similarly, in U.S. Pat. No. 4,649,348 to Flugan, a receiving coil for use in NMR imaging is constructed which, while having a high quality factor (Q), suffers from a poor signal to noise ratio (SNR). Other examples of rigid surface coils are exemplified in U.S. Pat. No. 4,692,705 to Hayes wherein embodiments of coils comprising a plurality of conductive segments and capacitive or inductive elements are illustrated. These types of coils are necessarily rigid, are usable only for very specific applications, and provide little opportunity for use in diversified areas. They do not satisfy the long felt need in the NMR art for coils which can be conformed to the same areas of different patients since the human anatomy can differ greatly in size from patient to patient. Thus, these coils cannot attain a consistently high SNR and provide ease of use and patient comfort.
It has been known in the NMR art that the closer the coil is to the sample or area of interest the better the image obtained. This occurs, in part, since the closer the coil is to the sample, the higher the SNR of the received electromagnetic signal. Thus, it has been generally known in the art to produce surface coils for NMR imaging of the human body as evidenced by the teachings of J. F. Schenck et al. in an article entitled "High Resolution Magnetic Resonance Imaging Using Surface Coils," Magnetic Resonance Annual, p 123 (1986) A "surface coil" is understood by those skilled in the art to be an NMR coil which is placed either directly on, or very close to, the area of the patient which will be imaged.
Considerable effort has gone into improving the SNR of NMR surface coils. It has been desired to create NMR surface coils that produce a consistently high SNR for all applications and clinical situations. However, this has heretofore not been achieved in the NMR coil art. Rigid, nonconforming surface coils are unable to achieve this desirable result since they cannot be placed close enough to the examination area to eliminate extraneous electromagnetic noise. Extraneous electromagnetic noise comes from the patient's own thermal radiation and various species of chemicals present in the patient which are responsive to the different magnetic fields applied during the NMR procedure.
NMR coils which attempt to bring the sample area close to the coil itself have been utilized so that a higher SNR can be achieved. Examples of such coils can be found in U.S. Pat. No. 4,680,549 to Tanttu and U.S. Pat. No. 4,587,493 to Sepponen, for example. The coil arrangement embodied in Sepponen cannot fairly be characterized as a "surface coil" since, rather than placing the coil near to the patient, the coil's Volume is modified to surround the area of the patient's anatomy which will be imaged. Such an arrangement is cumbersome and is strictly limited by the maximum volume achievable with the coil. Similarly, the physical dimensions of the coil arrangement in Tanttu severely limit its ability to be placed near enough to the patient's body to achieve a high SNR. Accordingly, there has been a long felt need in the art for surface coils which can be placed directly on the surface area of the patient's anatomy to be imaged so as to achieve a high SNR. This need has not been fulfilled by any known prior art coil.
Various coils have been utilized which attempt to solve the problem of extraneous electromagnetic noise. In general, surface coils which have some degree of flexibility in parts of their elements allow the coils to conform to a certain extent to the area of examination. An example of this type of coil is disclosed in U.S. Pat. No. 4,398,149 to Zens. Zens discloses therein that the coils may be printed on flexible insulating materials. Similarly, in U.S. Pat. No. 4,646,024, Schenck et al. disclose that NMR coils can be disposed on a flexible printed circuit board material which is then wrapped around a coil of appropriate diameter to form the apparatus. However, neither of the inventions embodied in these two patents satisfy the long felt need for flexible conforming surface coils with a high SNR. Although the coils described in both of these patents are printed on circuit boards comprised of electrically insulating materials which are somewhat flexible, their degree of flexibility is substantially less than necessary to provide the ability for these coils to completely conform to the area of the patient's anatomy which will be imaged. Furthermore, the coils disclosed in the Schenck et al patent are in fact rigid since the flexible circuit board material is wrapped around a stiff cylinder to complete the coil apparatus. Thus, neither of the coils embodied in the patents to Zens and Schenck et al satisfy the long felt need in the NMR art for surface coils which closely conform to a patient's anatomy and which can be used interchangeably among patients whose anatomies vary in size.
U.S. Pat. No. 4,636,730 to Bottomley relates to other coils which are positioned on relatively flexible substrates to allow conformity to the sample to be investigated. In particular Bottomley discloses a flexible insulative substrate which forms the body of a printed circuit board upon which the NMR coil is constructed, thereby giving the coil some ability to conform to the sample to be imaged. As with the Zens and Schenck et al. coils, Bottomley discloses relatively thin surface coils which have been constructed on printed circuit boards having some degree of flexibility associated with the substrate material. However the device embodied in the Bottomley patent does not fulfill the long felt need in the NMR coil art for surface coils which are adjustable for a multitude of patients and which closely conform to a patient's anatomy thereby allowing formation of an image having a high SNR, for the surface coils disclosed by Bottomley are only partially conforming since they are also formed on printed circuit boards having only a modest degree of flexibility.
The various inventions embodied in the above-cited patents fail to provide NMR surface coils which are highly flexible and conforming. However, in U.S. Pat. No. 4,617,936 Malko discloses flexible surface coils for use in magnetic resonance imaging which are comprised of an electrically insulating flexible tube which is filled with liquid mercury. The liquid mercury within the flexible tube detects the resonant signals from the area of the patient's body which is under observation since mercury is electrically conductive. However, the coils embodied in the Malko patent have several serious disadvantages. In particular, the use of liquid mercury as a conductive element to receive the resonant signals from the patient is dangerous since mercury is highly toxic. Thus, should the insulating tube rupture, the patient would be exposed to poisonous mercury immediately adjacent to a potentially vital area of his anatomy. Moreover, regardless of the type of conducting liquid used, a slow leak would significantly reduce the ability of the coil to detect the resonant signals from the patient. Additionally, the coils disclosed by Malko do not fully conform to the area of the patient's anatomy to be imaged since the tube can only be loosely wrapped around the desired area.
All of the NMR coils embodied in the aforementioned patents produce low quality images due to the existence of artifacts and aliasing. "Artifacts" are signals which are detected by the coil but have sources from other than those particular areas of the patient which are under observation. Since the coils described above cannot truly conform to the particular area of the patient under observation, they detect significant artifacts from parts of the patient's body which are not of interest. On the other hand "aliasing" is a phenomenon which exists in all forms of electromagnetic signal detection and processing. Generally, aliasing (sometimes referred to as "foldover and aliasing") takes place when a continuous signal that is not bandlimited is detected by an NMR coil such as those described. Thus, images from features outside the field of view (FOV) of the coil are superimposed upon features within the FOV of the coil. Aliasing occurs when high frequency signals foldover into the frequency region of interest and masquerade as the low frequency signals of interest. All of the coils described above exhibit a large degree of aliasing since they are not able to closely conform to the area of interest and thus detect signals emanating from areas outside of the desired FOV of the coil. Aliasing contributes to low quality resolution by distorting the images of the area of the patient under observation.
There is thus a long felt but unfulfilled need in the magnetic resonance imaging art for flexible surface coils. This long felt need requires coils which can closely conform to the patient's body and which reduce the detection of artifacts and limit distortion due to aliasing. An additional long felt need exists in the art for surface coils which have a high SNR and that can be placed directly on the area of the patient which is to be imaged. It is desirable that such coils be constructed of flexible materials that are able to closely conform to a specified area of a patient's anatomy and provide a high degree of comfort to the patient. NMR coils which would fulfill these needs should be adjustable so that patients whose anatomies vary in size can be imaged with the same coil. Similarly, a long felt need in the art exists for flexible, conforming surface coils which are usable with existing NMR magnets and support systems to ensure efficiency and economy.